1. Field of the Invention
The present invention relates to an image forming apparatus using an electrophotographic method, such as a copy machine, a printer, a facsimile machine, and a multifunction peripheral integrating the functions of these apparatuses.
2. Description of the Related Art
In a color image forming apparatus using an electrostatic method, image formation is performed by the well-known electrophotographic process in which toner (developer) images are formed on surfaces of photosensitive drums for respective colors, and the toner images of the respective colors on the photosensitive drums are transferred to a recording sheet via an endless belt-like intermediate transfer member. Drive sources for driving the plurality of photosensitive drums for rotation are generally implemented by a single kind of motors (e.g. brushless DC motors or stepper motors). Particularly, a brushless DC motor as an outer rotor-type motor is often employed from the viewpoint of rotational stability. The reason for this is as follows:
(1) Compared with an inner rotor-type motor, the moment of inertia of the rotor itself can be increased, and rotation fluctuations caused by the motor are less liable to be transmitted to the load side (photosensitive drum) when the rotational speed is not lower than a predetermined rotational speed.
(2) Even when load fluctuation is generated, the load fluctuation is suppressed by an amount corresponding to a gear reduction ratio by a speed reducer, and at the same time, the rotation fluctuation can be suppressed by the flywheel effect of the rotor.
(3) By controlling the motor drive by a PLL control method, it is possible to improve the rotational stability.
As mentioned above, the outer rotor-type brushless DC motor has above advantages (1) to (3), but on the other hand, a start-up time and a stop time of the motor sometimes vary depending on load torque. Particularly, in an image forming apparatus which drives a plurality of photosensitive drums by respective separate brushless DC motors, this problem brings about a fluctuation in rotational phase between the respective photosensitive drums.
As a countermeasure to differences in rotational phase between the respective photosensitive drums, there has been proposed e.g. a method in which a toner patch as a reference is formed on each of the photosensitive drums, and an optical sensor reads a result of transfer of the toner patches on the respective photosensitive drums to the intermediate transfer belt, thereby correcting the differences in rotational phase. There is also proposed a method of performing feedback control using a rotational speed-detecting unit provided on each photosensitive drum shaft, to thereby stabilize the rotational speed of the photosensitive drums. This method, however, employs not the PLL control method which requires rotational stability of a motor output shaft but a control method which is capable of variably controlling the motor rotational speed.
As described above, there have been proposed various kinds of methods for the electrophotographic image forming apparatus with a view to improving image quality. However, all the methods are effective only when the photosensitive drums of the respective colors have the same diameter.
In recent years, for the purpose of improving productivity and the like, there has been proposed an image forming apparatus that employs different diameters for a photosensitive drum for black and photosensitive drums for the other colors. In such an image forming apparatus, if driving sources of the respective photosensitive drums are implemented by motors of the same type, this requires a reduction gear ratio of each speed reducer (e.g. the number of reduction gears) to be changed. As a result, the ranges of rotational speeds toward the motor side become largely different, which sometimes makes the influence of motor-side rotational fluctuation on the load side (photosensitive drums) conspicuous, or causes rotation fluctuation due to load fluctuation. To improve such a situation, there has been proposed a technique that improves image quality by using motors of a plurality of types instead of the motors of the same type (see e.g. Japanese Patent Laid-Open Publication No. 2007-47629).
In an electrostatic color image forming apparatus disclosed in Japanese Patent Laid-Open Publication No. 2007-47629, when the same color stability of a color image as reproduced by an offset printing machine is required, it is necessary to always keep the same phase relationship between the photosensitive drums. As a result, to make the photosensitive drums in phase with each other, the photosensitive drum for black is driven by an outer rotor-type motor, and the photosensitive drums for the other colors are driven by an inner rotor-type motor, whereby the motors of different types are mixedly used.
Further, Japanese Patent Laid-Open Publication No. 2007-47629 describes that a brushless DC motor as an outer rotor-type motor has the advantage of contributing to stabilization of rotational speed, but has the disadvantage of a rotational angle at the start of rotation or at the stop of rotation being liable to vary depending on the load torque. As a result, Japanese Patent Laid-Open Publication No. 2007-47629 proposes employing an arrangement in which the photosensitive drums other than the photosensitive drum for black are each driven by a stepper motor as an inner rotor-type motor, thereby preventing color misregistration by phasing and facilitating the color misregistration prevention.
In the case of the arrangement in which a plurality of photosensitive drums and an intermediate transfer member are separately driven, if the brushless DC motors as outer rotor-type motors are employed, the brushless DC motor has the above-mentioned disadvantage of a rotational angle at the start of rotation or at the stop of rotation being liable to vary depending on the load torque. That is, if the level of load is different between the respective drive sources, there is caused a difference in the change of the rotational speed when starting or decelerating the motors, which generates a difference in speed between the photosensitive drums and the intermediate transfer member, and as a result, this causes scratches on the surfaces of the photosensitive drums and also causes image deterioration. To solve such a problem, there has been proposed an improving method employing speed profile definitions at the start and stop of motors, gain adjustment, and braking control (see e.g. Japanese Patent Laid-Open Publication No. 2003-091128).
In Japanese Patent Laid-Open Publication No. 2003-091128, the stepper motors are each subjected to speed control using the same start and stop profile, and the brushless DC motor is subjected to current control such that a speed change equivalent to that in each stepper motor is caused, by performing position and speed detection using an encoder.
As described in Japanese Patent Laid-Open Publication No. 2007-47629, when the stepper motor and the brushless DC motor are used in combination as the drive sources for the plurality of photosensitive drums and the intermediate transfer member, this brings about the following two problems:
(1) Occurrence of a displacement of a rotor due to a change in torque of the stepper motor
As shown in FIGS. 6A and 6B, when constant current control which is generally employed in the method of driving a stepper motor is performed, as a unique characteristic of the stepper motor, the position of the rotor of the motor changes depending on the load torque. That is, although speed of the stepper motor is controlled according to the frequency of an input speed command signal (pulse signal), if a change is caused in the load torque, displacement in the rotor and speed fluctuation caused by the displacement are caused, and as a result, color misregistration in a generated image is caused. Particularly, when the stepper motor is employed for the drive source for the photosensitive drum for black, the reduction ratio is not large (i.e. influence of the displacement on the motor shaft side does not become small), and at the same time the outer diameter of the photosensitive drum is larger than that of the other photosensitive drums. Therefore, this causes a problem that displacement in the angle through which the motor shaft rotates is likely to affect the displacement of the surface of the photosensitive drum.
To prevent such displacement of the position of the rotor due to a change in the load torque, it is necessary to increase the exciting current supplied to the stepper motor. However, this causes an increase in power consumption and a rise in the temperature of the motor.
(2) Generation of a difference in speed between the stepper motor and the brushless DC motor at the start-up time, and an increase in the difference in peripheral speed between the photosensitive drums and the intermediate transfer member and an increase in torque, which are caused by the difference in the speed between the motors.
Although the brushless DC motor is subjected to current control by a feedback control method for speed control such that as the difference in actual rotational speed from the set speed is larger, acceleration is increased, the acceleration is not always constant due to the load torque. For this reason, in general, a large difference in the acceleration may be generated between the brushless DC motor and the stepper motors subjected to an open-loop speed control. As a result, the peripheral speed difference from the intermediate transfer belt causes a large change in the load applied to each stepper motor, which causes a problem that the stepper motor suffers from a loss of synchronism at the start-up time. Further, also on the brushless DC motor side, a torque increase caused by a reaction force brings about an increase in supply current or an increase in the start-up time.
Japanese Patent Laid-Open Publication No. 2003-091128 proposes a technique for preventing a speed difference between the motors of the same type (e.g. between only the brushless DC motors or between only the stepper motors). However, the document discloses no discussion about a method of reducing a difference in drive characteristics between different types of motors.